Current radiocommunications systems conventionally use, for the transmission of digital data coding an audio signal or, more generally, information of any kind, so-called constant envelope modulations. With such modulations, the data transmitted are not carried by the amplitude of a radiofrequency carrier but by its phase or its frequency.
However, it is currently sought to transmit more information within a frequency band of given width, assigned to a transmission channel, in such a way as to increase the spectral efficiency of radiocommunications systems. The aim is to respond to the increase in traffic demand within the radiofrequency spectrum, while complying with the constraints related to the sharing of this spectrum. This is why reintroduction of amplitude modulation is envisioned, in addition to phase or frequency modulation. Thus, one seeks to devise new radiocommunications systems using, for the transmission of information, composite modulation, comprising both a phase modulation component and an amplitude modulation component.
Despite this, because of the need to maintain considerable power efficiency of the transmitter (which is particularly necessary within the context of use of the transmitter in a handheld radiocommunications appliance), the radiofrequency power amplifier is still operated in an operating zone close to saturation.
Now, as is known, in such an operating zone, the transmitter exhibits nonlinearities of amplification comprising amplitude nonlinearities and phase nonlinearities. In the literature, these amplitude and phase nonlinearities are often designated by the terms amplitude/amplitude conversions (or AM/AM conversions) and amplitude/phase conversions (or AM/PM conversions) respectively. These nonlinearities give rise to amplitude distortion and phase distortion of the signal transmitted, which degrade the performance of the transmitter in terms of transmission quality.
It is therefore desirable to cancel out the effects of the amplification nonlinearities induced by the radiofrequency power amplifier, so as not to degrade the quality of transmission. Several techniques are known, that allow this result to be obtained.
In the CLLT (standing for “Cartesian Loop Linear Transmitter”) technique, the signal to be transmitted is firstly generated in baseband. A modulator provides for the conversion of the baseband signal to the radiofrequency domain (upconversion). Finally, the radiofrequency signal is amplified in the power amplifier. A coupler followed by a demodulator make it possible to tap off part of the radiofrequency signal at the output of the power amplifier and to convert it to baseband (downconversion). The signal generated in baseband is compared with the signal thus demodulated in baseband, by means of a comparator whose output drives the modulator. Analog feedback control of the transmitted radiofrequency signal by the signal generated in baseband is thus performed. This feedback control makes it possible to cancel out the nonlinearities present in the upconversion chain, in particular the nonlinearities induced by the modulator and by the radiofrequency power amplifier. The linearization performance of this technique is however limited by the passband of the feedback control loop. For problems of stability of the feedback control loop, the loop gain is in fact generally small, thereby resulting in a correction of the limited nonlinearities. Usually, it is necessary to include a detector of instability of the feedback control loop. Furthermore, the spectral purity is limited by the presence of a conventional modulator in the upconversion chain.
In the ABP (standing for “Adaptive Baseband Predistortion”) technique, adaptive predistortion is applied to the signal generated in baseband, generally by digital processing. The predistorted signal is generated in baseband, via a digital/analog converter. Then, the predistorted signal is converted to the radiofrequency domain (upconversion) by virtue of a modulator. Finally, the radiofrequency signal is amplified in the radiofrequency power amplifier. The predistortion makes it possible to cancel out the nonlinearities present in the upconversion chain, in particular the nonlinearities induced by the modulator and by the radiofrequency power amplifier. This is an adaptive predistortion. For this purpose, a coupler followed by a demodulator make it possible to tap off part of the radiofrequency signal transmitted and to convert it to baseband (downconversion). The signal demodulated in baseband is digitized and compared with the signal generated in baseband (desired signal). Adaptation of the predistortion coefficients then enables the signal thus demodulated to be made to converge to the desired signal. The downconversion chain therefore allows regular updating of the predistortion coefficients. This technique requires a learning phase, during which a degradation in the spectral purity of the transmission is very often permitted. The spectral purity is furthermore limited by the presence of a conventional modulator in the upconversion chain.
In the EER (standing for “Envelope Elimination and Restoration”) technique, the principle of which is illustrated by the diagram of FIG. 1, the modulation of the radiofrequency signal to be transmitted is decomposed into a phase or frequency modulation component and an amplitude modulation component. These two components are generated in baseband. The phase or frequency modulation component drives a phase or frequency modulator MOD (for example a duplicating loop), that provides for the conversion of this component to the radiofrequency domain. The output signal from this modulator is a phase- or frequency modulated signal of substantially constant amplitude. This signal is amplified by virtue of the radiofrequency power amplifier PA. The amplitude modulation component is used, via adaptation circuits (not represented), to control the gain of the power amplifier PA. The power amplifier PA may be a hardware item comprising a gain control input or an assembly of hardware items comprising a gain control input. Thus, the amplitude modulation component is superimposed on the phase or frequency modulation component to obtain the desired radiofrequency signal at the output of the radiofrequency power amplifier PA. These two components use different paths to reach the output of the amplifier. To cancel out the nonlinearities, in particular those induced by the radiofrequency power amplifier PA, it is necessary to introduce devices for correcting these nonlinearities. A loop for analog feedback control of the amplitude modulation may be introduced, as shown in the diagram of FIG. 2, which will be described in detail later. This feedback control loop makes it possible to compensate for the amplitude nonlinearities. To compensate for the phase nonlinearities, it is possible to introduce, as for the CLLT or ABP techniques, a downconversion chain comprising a demodulator.
However, such a demodulator comprises a local oscillator for performing the conversion from the radiofrequency domain to the baseband. This local oscillator, not modulated, is at the same frequency as the signal to be transmitted. This raises complex problems of decoupling between this local oscillator and the signal to be transmitted, in particular for the radiofrequency transmitters of mobile stations by reason of the constraints related to their compactness. Moreover, the synthesis spacing of this local oscillator is equal to the channel spacing of the radiocommunications system concerned. This local oscillator is moreover a complex and bulky element.